A condenser microphone has a diaphragm made of a resin film and having a metallized film on one surface set up on a supporter ring (diaphragm ring) to render it as a diaphragm assembly, forms a condenser by placing the diaphragm assembly and a fixed pole oppositely via a spacer and converts a sound wave to an electrical signal according to change in capacitance due to displacement of the diaphragm oscillated by the sound wave.
Direct-current bias voltage is applied to this converter (except a microphone of an oscillation detecting method). Sensitivity of the condenser microphone depends on the bias voltage, and the sensitivity can be increased by increasing the bias voltage.
However, electrostatic attraction acts on the diaphragm between itself and the fixed pole. Therefore, if the bias voltage over a limit of stability is applied, the diaphragm is attracted to and eventually contacts the fixed pole side so that the microphone becomes no longer operational. In that case, a central portion of the diaphragm contacts the fixed pole first. If the bias voltage is further increased, its contact range expands to a periphery of the diaphragm.